This invention relates generally to the formation of thin films by sputter-deposition and is particularly directed to the sputter-deposition of thin films of precisely controlled composition and dimensions using a single ion, electron or laser beam and a multicomponent target.
Immediately after the discovery of high T.sub.c bulk superconductor materials at 40 K and 90 K, an intense research activity was initiated to develop processes for the fabrication of high T.sub.c thin film superconductors. Several techniques have been demonstrated for the deposition of these films, including: (a) electron beam-induced coevaportation and multilayer deposition of elemental Y, Ba, and Cu, (b) plasma and ion beam sputter-induced deposition from irradiated bulk superconductors, (c) pulsed laser evaporation of bulk superconductors, and (d) molecular beam epitaxy. To date, the ion beam sputter-deposition method has been demonstrated only with targets consisting of bulk superconductors, e.g., La.sub.1.8 Sr.sub.0.2 CuO.sub.4 and YBa.sub.2 Cu.sub.3 O.sub.7-x.
In general, ion bombardment-induced sputtering produces ejection of surface atoms via ion-induced collision cascades in the solid, the ejected atoms having energy distributions with peaks at 2-5 eV for normal incidence bombardment and up to several hundred eV for bombardment at oblique angles of incidence. This may present an advantage over vaporization methods mentioned above in that the higher energy of the sputtered species may result in implantation into the substrate, producing improved adhesion of the thin film. A problem related to sputtering of multicomponent materials (alloys and compounds) is that preferential sputtering may lead to compositional changes to a certain depth below the surface of the sputtering target. The composition of the deposited films will therefore vary with sputtering time. This is, however, a transient condition since a steady-state composition will be produced in which the sputtering rate of the constituent species is proportional to the bulk composition. Unfortunately, in some cases, a relatively thick layer (.about.0.5 .mu.m) must be removed before the steady-state condition is reached. The stoichiometry of sputter-deposited films, prior to the annealing process necessary to produce the superconducting phase, has been shown to be in general not the same as that of the bulk material used as the target. Additionally, the films have shown, in general, lower T.sub.c 's than those characteristic of the bulk superconductors. This appears to also be the case for laser-deposited films. Two possible reasons for the lower T.sub.c values for the thin films could be a failure to exactly reproduce the stoichiometry of the bulk superconducting target and/or interdiffusion at the film-substrate interface during the annealing process. Recent studies have shown first evidence for laser and ion bombardment-induced compositional changes in YBa.sub.2 Cu.sub.3 O.sub.7-x targets which may be responsible for variations in film stoichiometry and for annealing-induced compositional changes in Y-Ba-Cu-O films.
In addition, strong surface-topographical changes have been observed in ion- and laser-irradiated bulk single phase YBa.sub.2 Cu.sub.3 O.sub.7-x targets, the development of a dense forest of large cones being the most noticeable effect. Referring to FIG. 1a, there are shown cones developed on a YBa.sub.2 Cu.sub.3 O.sub.7-x single phase superconductor as a result of several hundred pulses of an XeCl excimer laser (.lambda.=0.308 .mu.m, 0.3J) during deposition of a high T.sub.c film. Shown in FIG. 1b is the sputter-induced (10 keV Kr.sup.+) cones on a single phase YBa.sub.2 Cu.sub.3 O.sub.7-x superconductor target. It should be noted that the texture of the cones shown in FIG. 1a is different from that of the sputter cones shown in FIG. 1b which is indicative of a melting-resolidification process. The sputter cones shown in FIG. 1b are characteristic of the well-known collisional sputtering-induced mechanism of cone formation.
The electron beam induced evaporation and molecular beam epitaxy techniques, on the other hand, have an advantage over the currently used ion sputtering and laser evaporation induced deposition methods in that the elemental components, e.g., Y, Ba and Cu, can be vaporized to deposit films with controllable stoichiometry. This capability could be significant in that a slightly copper-rich phase of the Y-Ba-Cu-O or other oxide may provide a higher T.sub.c, and it is therefore desirable to be able to make films with composition differing controllably from the YBa.sub.2 Cu.sub.3 O.sub.7-x stoichiometry. However, presently used configurations for electron beam evaporators and molecular beam epitaxy equipment require the use of either one electron gun or one oven for each component of the thin film. This approach results in increased cost and complexity of the equipment and also imposes the requirement of accurately matching the characteristics of all sources.
The present invention represents an improvement over the aforementioned prior art approaches through the use of a computer controlled, single ion beam with a quartz crystal monitor to produce deposited films of arbitrary composition as well as layered structures of arbitrary thickness. The present invention thus provides an alternative approach for the synthesis of high T.sub.c superconducting oxides and other multicomponent films, e.g., GaAs-AlGaAs superlattices, multicomponent oxides for opto-electronic applications, etc., which can be applied not only to the ion beam sputter-deposition method, but also to the electron and laser beam-induced vaporization methods.